Concentration type solar heat plants are roughly classified into independent plants and Hybrid plants. In the independent plants, most heat is provided by solar heat while a part of the heat is backed up by fossil fuel or the like. On the other hand, in the Hybrid plants, most heat is provided by fossil fuel or nuclear fuel while a part of the heat is backed up by solar heat.
In both types of the independent plants and the Hybrid plants, heat from sunlight is collected and used as a heating source, and a solar collector is also used substantially in common. Generally, a trough type light/solar collector (in which a parabolic mirror is provided and a heat transfer pipe is disposed at a focal point where the sunlight is focused), a Fresnel type solar collector (in which a large number of flat mirrors are provided and heat transfer pipes are disposed at focal points where the sunlight is focused) and a tower type solar collector (in which a large number of flat mirrors (hereinafter also referred to as mirrors simply) are placed in a wide region close to the ground surface and a heat transfer panel serving as a solar collector is disposed at a focal point where the sunlight reflected by the mirrors is focused) can be used as the solar collector.
Among them, the trough type solar collector and the Fresnel type solar collector are short in the focal length and low in the degree of concentration of the sunlight (the heat density in a heat collection portion). On the other hand, the tower type solar collector is long in the focal length to be able to use light reflected from a wide region. Thus, the tower type solar collector has characteristic that the degree of concentration of the sunlight (the heat density in a heat collection portion) is high. When the heat density in the solar collector is high, the amount of collected heat per unit heat transfer area is large so that higher-temperature steam can be obtained.
Next, an example of a tower type independent plant will be illustrated for explaining a background-art technique. FIG. 7 shows a schematic view of a typical tower type independent plant. By a feedwater pump 11, water is delivered to a solar collector 13 placed on a solar collector supporting base 12. On the other hand, light from the sun 14 is reflected by mirror surfaces of heliostats 15 constituted by mirrors and drive units, and collected onto the solar collector 13. In the solar collector 13, the temperature of the water rises due to the heat of the sun so that steam is generated. The steam generated in the solar collector 13 is delivered to a steam turbine 16. The steam turbine 16 is driven so that a generator 17 can generate electricity.
As an example of the structure of the solar collector 13, FIG. 8 shows an example of a solar collector including an evaporator 19 and a superheater 20. Water supplied by the feedwater pump 11 (see FIG. 7) once enters a steam-water separation device 18. The water is circulated and heated in the evaporator 19 so as to turn into steam partially, and then enters the steam-water separation device 18 again. In the steam-water separation device 18, the water is separated into saturated steam and saturated water. The saturated steam enters the superheater 20, while the saturated water enters the evaporator 19 again. The saturated steam entering the superheater 20 is heated by solar heat so that superheated steam is generated. The generated superheated steam is delivered to the steam turbine 16 (see FIG. 7).
FIG. 9 schematically shows the structure of the superheater 20 (heat collection portion) in the aforementioned solar collector 13 and the distribution of a thermal load (unit: KW/m2) in the superheater 20 (heat collection portion). Each broken line connects portions of the same thermal load like a contour line and shows each region of the same thermal load stepwise.
Saturated steam a generated in the evaporator 19 is supplied to the superheater 20 (heat collection portion) through an inlet header 1 and heated by solar heat so as to turn into superheated steam d, which flows out through an outlet header 5. The pattern shape of incident light reflected by each mirror with a fixed shape and reaching a light receiving surface serving as the heat collection portion of the solar collector is changed by the azimuth angle and the elevation angle of each heliostat following the sun in accordance with the positional relation between the mirror and the light receiving surface and the azimuth and the altitude of the sun.
For example, even if a square mirror is used, the pattern of an incident light ray may be changed to be longer horizontally than vertically or longer vertically than horizontally. In addition, the patterns of incident light rays reaching the light receiving surface from a plurality of mirrors having the same shape but placed in different positions are different from one another. Therefore, a distribution may occur in the intensity of incident light obtained by those incident light rays superimposed on one another. That is, a region high in thermal load and a region low in thermal load are generated, as shown in FIG. 9.
On the other hand, a portion where light rays reflected by a plurality of mirrors are superimposed (overlapped) becomes high in temperature. It is therefore undesirable that the reflected light rays which are incident on a portion other than the heat collection portion of the solar collector may thermally damage any member in that portion.
Therefore, during an operation period of the solar heat boiler (solar collector), the region of the light receiving surface of the solar collector (i.e. the width and height of the heat collection portion of the solar collector) is generally set to be larger than the shape of incident light (hereinafter also referred to as whole incident light pattern) in which incident light rays from all the mirrors in use are superimposed, so that the whole incident light pattern can be prevented from protruding from the light receiving surface of the solar collector.
The upper/lower range or distribution of the absolute value of the thermal load of the superheater 20 (heat collection portion) fluctuates depending on the installation conditions (dimensions, shapes, installation region, number, etc.) or the tracking method (control) of the heliostats (mirrors) and depending on the azimuth or altitude of the sun. However, the thermal load is highest in a central portion (high thermal load region e) of the superheater 20 and decreases as it goes more closely to the periphery (low thermal load region f) of the superheater 20.
FIG. 10 shows the relation between the width-direction (direction perpendicular to the axis direction of each heat transfer pipe) position of the superheater 20 (heat collection portion) shown in FIG. 9, which position is on the abscissa, and a temperature difference of a fluid between the inlet header 1 and the outlet header 5 in the superheater 20 (heat collection portion), which temperature difference is on the ordinate.
The heat transfer pipe (the reference sign g shown in FIG. 9) in the central portion is located in the high thermal load region e. For example, on the conditions in which a fluid to be heated (saturated steam a) flows into the inlet header 1 and at a pressure of 5 MPa and the fluid (superheated steam d) flows out from the outlet header 5 and at an average temperature of 500° C., the temperature of the fluid reaches about 600° C. at the outlet portion while the temperature of the fluid to be heated (saturated steam a) is a saturation temperature of about 250° C. at the inlet. Thus, the temperature difference of the fluid between the inlet header and the outlet header is about 350° C. On the other hand, the heat transfer pipes (the reference sign h shown in FIG. 9) in the opposite side portions are located in the low thermal load region f. Accordingly, the heating amount is so low that the temperature of the fluid reaches about 350° C. at the outlet portion while the temperature of the fluid to be heated (saturated steam a) is about 250° C. at the inlet. Thus, the temperature difference of the fluid between the inlet header and the outlet header is about 100° C. As a result, the width-direction temperature difference in the outlet header reaches about 250° C. (600° C.-350° C.). In this manner, a large temperature difference arises among width-direction positions of the superheater (heat collection portion).
In addition, for example, Patent Literature 1 discloses a boiler using solar heat, in which a plurality of solar heat light reception panels each having a lower header and an upper header are connected in cascade so that a fluid passing through pipes can be heated sequentially. This Patent Literature 1 also has no particular suggestions about heat reception properties in width-direction positions of the light reception panels.
In addition, Patent Literature 2 discloses an arrangement structure of heat transfer pipes as follows. That is, a lower header is provided in the width direction of a passage through which a heat medium as exhaust gas passes, and the lower header is divided into three portions by two partition plates so that a fluid from an inlet pipe can flow into a division port of a width-direction center portion and the fluid returning from the division port of the center portion via an upper header can flow into division ports on the opposite sides of the center portion. This Patent Literature 2 suggests that an inlet and an outlet are provided in the lower header so that a communication pipe heretofore placed between the upper header and the lower header can be eliminated.